Uranium Spectral Gamma‐ray Response as a Proxy for Organic Richness in Black Shales: Applicability and Limitations

In many organic‐rich, low‐carbonate hemipelagic shales, there is a stable and close correlation between the uranium and TOC contents. In this paper, we present a number of case studies using our own data and that from previous publications to investigate black shales with (1) good, (2) fair‐to‐good and (3) poor U/TOC correlations. U/TOC ratios in the different black shale units are compared to each other, and possible reasons for the observed variations are discussed. In general, the U/TOC ratio in a black shale is controlled by a number of factors which include for example the primary uranium content of the water body, the carbonate content and the sedimentation rate. The development of a stable U/TOC ratio may be inhibited by the presence of phosphate, by a high carbonate or sand content, by dissolution (“burn‐down”) of uranium during intermittent oxic periods, and by large‐scale diagenetic remobilisation of uranium. In suitable black shale systems, vertical variations in organic richness can be approximated by measuring the uranium content using spectral gamma‐ray measurements. This may be especially important in outcrop studies because gamma‐ray logging is a straightforward field technique. Before the uranium content can be used as a proxy for TOC content in a black shale system, however, a thorough calibration of uranium and TOC is necessary, in order to determine the stratigraphic and regional limits of the derived U/TOC ratios and to establish the presence of a stable U/TOC correlation.